Carrier 59SC5A Error Code Continuous OFF: No Power / Control Failure

What Does Code Continuous OFF Mean?

When the status LED on your Carrier 59SC5A is continuously off, the HK42FZ036 control board is not receiving power at all. Unlike a blink code, which the board can only display when it is powered and running its diagnostics, a dark LED means the board itself is dead. The furnace cannot do anything in this state because the HK42FZ036 manages every stage of operation, from the inducer and igniter to the blower.

Carrier's service label directs a technician to check for 115 VAC across L1 and L2 (the incoming line voltage) and 24 VAC across SEC-1 and SEC-2 (the transformer's secondary output). Working outward from the homeowner-safe causes, the most common reason is a tripped breaker or blown fuse at the panel, followed by the furnace power switch being off. If both of those are fine, the fault is usually a failed transformer or the board itself, which is professional territory.

This is different from the steady 'Continuous ON' indication, where the LED is lit and the board is simply in standby. Continuous OFF is a no-power condition; Continuous ON is a healthy, powered board waiting for a call for heat.

What You'll Notice

• The amber status LED on the control board is completely dark, with no steady glow and no blink pattern
• The furnace is unresponsive to the thermostat and produces no heat at all
• The inducer motor, hot surface igniter, and blower never start
• If a breaker tripped, other devices on the same circuit may also be without power

Common Causes

How This Is Diagnosed

The fault is isolated from the incoming power inward. First the breaker or fuse for the furnace circuit is checked, then the dedicated furnace power switch, since both cut 115 VAC to the unit. If line voltage is confirmed present at L1 and L2 but the board is still dark, a technician then measures the transformer's 24 VAC output at SEC-1 and SEC-2; no 24 VAC with good 115 VAC in points to a failed transformer or a blown onboard fuse, and a dead board with good 24 VAC points to the control itself. The voltage measurements require a meter and are not homeowner steps.

How to Fix It: Check the Circuit Breaker and Furnace Power Switch

What You'll Need

• Flashlight

Steps

1. Turn off the furnace power switch and shut off the gas supply before investigating Flip the furnace power switch (a light-switch-style switch on or near the unit) to OFF, and turn the manual gas shutoff valve to the OFF position (handle perpendicular to the pipe) as a precaution while you work. If you smell gas, leave immediately and call your gas company.
2. Check the circuit breaker or fuse At your main electrical panel, find the breaker labeled for the furnace or HVAC. If it sits in the middle tripped position, flip it fully to OFF and then back to ON. If your panel uses fuses, check the furnace fuse and replace a blown one with the same amperage rating.
3. Turn the furnace power switch back on Return to the furnace power switch and set it to ON. This switch is easily mistaken for a room light switch and is a common reason a furnace loses power after someone works nearby.
4. Restore gas and verify the board powers up Turn the gas supply valve back to ON (handle parallel to the pipe). Look at the control board through the access panel: the LED should now be lit steady (Continuous ON), indicating the board has 24 VAC and is in normal standby. Set the thermostat to call for heat and confirm the furnace starts.

When to Call a Professional

Contact a licensed HVAC technician if:

• The breaker trips again immediately each time you reset it
• The breaker is not tripped and the power switch is on, but the LED stays dark
• You notice a burning smell, scorch marks, or other signs of electrical damage near the furnace
• You are not comfortable working around your electrical panel

✓ Verified against manufacturer service manual — March 2026